A few tips for hitting 225 or whatever temp you desire in your WSM. I usually like to target 225 for butts but will also run the WSM at 240, which turns out great pork.
1. Use Minion method and stick with lighting about 20 briquets.
2. Always shut down the bottom vents at 10 degrees below target temp. The temp typically continues to rise and, if not, you can always open them back up. It is much easier to increase the heat in the cooker than it is to lower the temp if running hot.
3. WSM's are sensitive to outside temp and direct sunlight so adjust the amount of lit and unlit charcoal accordingly. When cooking in direct sunlight in the summer, I use less unlit and lit charcoal. In the winter use more. When trying to run a low temp in the summer, put the cooker in the shade.
4. If the WSM is running hotter than your target temp, you can partially close the top vent if there isn't a lot of smoke coming out of the vent. I have closed the top vent up to a half or 3/4 for a few hours to get the temp down.